What is the theme of the book?

One major theme is the book is The Impossibility of Returning to Childhood. Near the end of the book, Enid attempts to return to what she sees believes was a happier time in her childhood, something she discovers is impossible. She begins this process in Chapter Seven while looking at the pictures in a photo album. Enid then becomes obsessed with finding a record that she used to listen to as a child, and when she can't find it, she visits several record stores unsuccessfully. The mere process of searching for the record seems to make Enid more emotional. This is important because while Enid usually protects herself with an emotional wall, she is unable to hide her emotions while searching for the record. Examples of this occur when Enid sees the old man taking flowers home to his wife, an action that almost reduces her to tears. And later that same evening, she goes over to Josh's apartment and confesses her feelings.

Enid continues trying to recapture her childhood in chapter eight when she takes a trip to Cavetown, USA with Rebecca. Enid claims that her trip to the park with her father and stepmother Joanie is her only happy memory from childhood. Enid is initially amused by the statues of dinosaurs and cavemen at Cavetown, USA but is also amazed by how small and unimpressive it all seems now. This indicates that although her childhood remains an important emotional touchstone, Enid cannot actually return to those feelings.